FR2652025A1 - APPARATUS AND METHOD FOR WELDING METAL PARTS BY LASER BEAM. - Google Patents

Abstract

This is a method and apparatus for laser beam welding of two metal parts (2, 41) together. The apparatus comprises an optical fiber (10) for transmitting the beam and a welding head (9) comprising focusing means (29) of the beam coming from the optical fiber, deflection means (37) of the beam coming from the means focusing (29) for focusing the deflected beam on the parts to be welded, a clearance zone (42) separating the deflection means (37) and the focusing point (39) of the beam, and gas scanning means sweep of this clearance area.

Description

The present invention relates to a laser beam welding apparatus

  of two metallic parts together, comprising an optical fiber for transmitting the welding laser beam and a welding head comprising means for focusing the

  laser beam from optical fiber.

  It also relates to a laser beam welding process of a metallic tubular sleeve inside a metallic tube of small internal diameter, for example of internal diameter between

  on the order of 10 mm and on the order of 50 mm.

  It finds a particularly important, although not exclusive, application in the field of repairing a leaking heat exchanger tube of the tube plate exchanger type, in particular at the level of the tube plate, the heat exchange taking place between the primary refrigerant of a reactor such as a pressurized water nuclear reactor or PWR, and water in liquid and vapor phases

  constituting the secondary refrigerant.

  It is important to be able to repair leaking tubes to avoid contamination by the primary circuit possibly loaded with radioactive elements, from the secondary circuit which must

imperatively remain inactive.

  Repairing a leaking tube, generally damaged or cracked at its attachment points with the tube plate, is difficult to perform; the internal diameter of a tube plate exchanger tube belonging to the cooling circuit of a nuclear reactor, is indeed

of the order of 20 mm.

  Welding devices of the prior art have drawbacks. They are too bulky, and / or require a great deal of energy consumption and are therefore at the origin of significant deformations at the level of the tubular plate and of the tube provided with its repair sleeve. So much so that the solution generally adopted to overcome the drawbacks of known welding devices, consists in isolating the leaking tube by closing it altogether, rather than seeking to repair it. This solution is also not satisfactory either because it has the drawback of reducing the number of tubes in

  service between primary and secondary circuits.

  The invention aims to provide a welding device and a method for welding a sleeve inside a small diameter tube, which meets the requirements of practice better than those previously known, in particular in that the device welding, while having a small footprint and excellent robustness, - allows for good quality welds, and in that the welding process is quick, easy to implement, and requires less consumption of energy that the processes

of the prior art.

  For this purpose, the invention essentially proposes an apparatus for welding by laser beam of two metal parts together, comprising an optical fiber for transmitting the beam and a welding head comprising means for focusing the beam coming from the optical fiber, characterized in that the welding head further comprises means for deflecting the beam from the focusing means, in order to focus the deflected beam on the parts to be welded, a clearance zone separating the deflection means and the focal point of the beam, and means for sweeping by a scavenging gas of this release zone arranged to drive the metal vapors out of the field of the deflection means

  from the fusion of metals during welding.

  The use of a laser beam to perform welds by deflected beam for example by transparency or pointing in tubes of small diameters, and requiring a welding head having means of deflection of the beam towards the parts to be welded, could not in particular not come to the idea of a person skilled in the art, due to the emission of metallic vapors instantly covering the deflection means during laser beam welding

  and thus preventing further welding.

  The present invention eliminates this

disadvantage.

  In advantageous embodiments of the invention, use is also made of one and / or the other of the following arrangements: the sweeping gas is a neutral gas, and advantageously argon; - The welding head is elongated around an axis substantially parallel to the direction of the axis of the laser beam from the optical fiber, and the beam deflection means comprise a plane mirror for returning the beam to the parts welding; - The beam deflection mirror is entirely located, relative to the focus point, at a distance substantially equal to or greater than the distance separating the axis of the welding head from said focus point; - The smallest distance separating the deflection means from the focusing point is less than around 30 mm and advantageously between around 5 mm and 10 mm. the end of the optical fiber from which the laser beam comes is fixed in the axis of the welding head, and the focusing means comprise optical means for deflecting the beam relative to the axis of the welding head to direct the beam on the deflection mirror; the focusing means comprise, arranged successively on the optical path of the beam coming from the fiber, a meniscus, a convex plane lens and the optical means for deflecting the beam, said optical deflection means being constituted by an asymmetrical lens formed by a cylindrical portion resulting from cylindrical trepanation or "coring", of a spherical convex plane lens in the vicinity of the axis of said spherical lens, the cylindrical coring being of axis parallel to said spherical convex plane lens. The use of this asymmetrical lens makes it possible to increase the space between the deflector and the point to be welded in order to better overcome the projections of metal vapors during welding. All optics are advantageously treated "anti-reflective" to the length

laser wave.

  - The gas scanning means, include deflectors constituted by walls arranged symmetrically with respect to a plane passing through the axis of the welding head and arranged to accelerate the speed of the gas in the release zone and increase the pressure dynamics exerted on the welding area; - The apparatus further comprises means for controlling and controlling laser welding in pulse emission mode of the latter during successive emission times of duration between about 10 milliseconds and about 50 milliseconds; the laser beam used is a beam emitted by a solid-body laser of the YAG laser type; - The apparatus further comprises means for automatic rotation of the welding head about its axis and means for automatic movement of said welding head along its axis, arranged to move said head longitudinally with respect to the parts to be

solder.

  The invention also provides a laser beam welding method of a metal tubular sleeve inside a metal tube with an internal diameter of less than 50 mm, characterized in that: - the sleeve to be welded is introduced into the tube with which it is made cooperating contiguously, (the sleeve introduced is advantageously fixed to the tube for example by swaging or by hydraulic expansion or crimping), - a welding head of elongated shape is introduced around an axis in the tube up to a determined position of the head relative to the sleeve to be welded, - a beam of laser radiation is emitted in a fiber optic system connected to the welding head by focusing the beam of laser radiation from the optical fiber on the sleeve portion to be welded by means of deflection means rigidly fixed to the head, which leads to the welding of the sleeve to the tube by fusion of the metal, and - it is swept simultaneously t when the laser beam is emitted, the welding zone by a sweeping gas injected inside the welding head and directed between the deflection means and said welding zone, to entrain the metallic vapors resulting from said melting of the metal outside the scope of

means of deflection.

  In an advantageous embodiment of the invention, the welding process is further characterized in that the welding head is rotated about its axis to produce a continuous or discontinuous circular welding of the sleeve on the internal wall of the tube , the laser emission being carried out in pulsed mode, and the interior of the tube is vacuumed simultaneously by the end of the tube opposite the end through which the welding head was introduced, in order to favor the evacuation of the neutral sweep gas injected inside said

head.

  However, the laser emission can also be

  performed in modulated or continuous regime.

  The invention will be better understood on reading

  the following description of embodiments

  particulars given by way of nonlimiting examples.

  The description refers to the drawings which

  accompany it in which: - Figure 1 is a block diagram of a welding device according to one embodiment of the invention, applied to the repair of a heat exchanger tube, - Figure 2 is a schematic axial section on a large scale of the welding head of the apparatus of FIG. 1, - FIG. 3 is a block diagram of part of the optical system of the apparatus of FIG. 1, - FIGS. 4a and 4b, show an alternative embodiment, respectively from the front and in section of a lens constituting the beam deflection means belonging to the focusing means of the apparatus according to the invention, FIG. 5 is a schematic view with partial section showing a part of the welding head of the apparatus according to an embodiment of the invention with lateral deflectors, - Figure 6 is a schematic section on VI-VI in top view of Figure 5, - Figure 7 is a partial vertical section according to

VII-VII of figure 2.

  The heat exchanger shown schematically in Figure 1 is of a type commonly used in pressurized water reactors. It comprises an envelope 1 which contains a bundle of U-shaped tubes 2, the end portions 3 of which pass through a tubular plate 4. The diameter of a tube 2 has been exaggerated in FIG. 1, in proportion relative to the envelope 1 and to plate 4. Plate 4 delimits with a partition a cold water box 6 and a hot water box 7, provided respectively with a flow pipe and a flow pipe for the primary coolant of the reactor (not represented). The part of the primary circuit contained in the exchanger is isolated and

drained in a manner known per se.

  Part 8 of the exchanger between the tubes 2 and the casing 1 is occupied, during operation, by the secondary refrigerant of the

  reactor and therefore belongs to the secondary circuit.

  The apparatus according to the embodiment of the invention more particularly described here, comprises a welding head 9 elongated around an axis, connected at one of its ends, to an optical fiber for transmitting the laser beam. The optical transmission fiber is preferably a silica monofiber with a diameter between 500 and 1500 μm. But other types of fibers, for example made up of several fibers (multifibres) can also be used. The fiber is connected at its other end to a source 12 of coherent radiation

  emitted by laser, of a type known per se.

  Advantageously, the laser is a solid laser of the type known under the name YAG (wavelength

  of the order of a micron) comprising Nd ions.

  The power density used with a laser source in an apparatus according to the invention is between 0.1 and 5 MW per cm2, advantageously of

around 0.3 MW / cm2.

  The welding head 9 is moreover connected via a hose 13 to a device 14 for supplying sweep gas, for example Argon, comprising a pressurized Argon tank and means regulating the flow rate of injected gas

  in the welding head 9 by the hose 13.

  The welding apparatus comprises means for automatic rotation of the welding head about its axis, and means for automatic longitudinal displacement of said welding head, along its axis. The rotation means (not shown) are constituted in a known manner for example by two pinions, one being integral with the head and the other with a support 15, meshing with one another around two parallel axes and operated by a numerically controlled stepping motor. The longitudinal displacement means (also not shown) are of known type and include for example an endless screw secured to the welding head cooperating with a nut fixed to the support 15 and actuated by a pitch motor

step by step.

  The support 15 is, for example, a machine commonly used during control and maintenance operations of a nuclear reactor, on which the welding head and its means of rotation and displacement are easily adaptable. The machine is advantageously of the type often called "spider". One will find possible constitutions of spiders by referring in particular to documents US-A-4018345 (COMBUSTION ENGINEERING) and FR A-2.309.314

(FRAMATOME).

  Means 16 for evacuating the interior of the tube 2 are provided. They comprise a sealed head 17 which is fixed on the opposite end 3 of the tube, relative to the end of the tube into which the welding head 9 is introduced. A hose 18 connects the head 17 to the vacuum means 16

  comprising for example a vacuum pump.

  The means for rotating and moving the head 9, the laser source 12, the supply devices 14 and the vacuum device 16, are controlled by command and control means 17. The means 17 comprise for example an automaton programmable and allow operation of

  the device as will be described below.

  Figure 2 is an axial section of the welding head 9 of the apparatus according to the invention more

particularly described here.

  The head 9 comprises a cylindrical casing 20 made of metal, for example stainless steel, around an axis 21; the outer diameter of the envelope is for example of the order of 19 mm. The envelope is therefore suitable for cooperating with the internal bore of the tube 2 of the exchanger, with an internal diameter slightly greater than 20 mm. The envelope 20 is open at one of its ends 22 and is provided at the other end 23 with a metal plug 24 pierced right through by passages 25 for sweeping gas supply, the path of which is shown in Figure 2 by arrows

  26 inside the welding head.

  The passages 25 are advantageously arranged symmetrically with respect to the axis 21 and are, for

example, 6 or 8.

  The welding head 9 is connected to the optical fiber 10 for transmitting the laser beam 27 through its end 28, fixed to the plug 24 in the axis 21 of

the welding head.

  As shown in FIGS. 2 and 3, the welding head comprises means 29 for focusing the beam 27, for example constituted by a spherical meniscus 30, a spherical convex plane lens 31 and optical means 32 for deflecting the beam constituted by a half convex plane spherical lens resulting from trepanation, by obviously cylindrical, of a convex plane lens of large

  diameter (broken line in Figure 3).

  More precisely, FIGS. 4a and 4b show an embodiment of the deflection means 32 constituted by a lens 35 with an axis 35 ′, resulting from the cylindrical "coring" or "trepanation" of a spherical convex plane lens 36, from axis 36 ', the axis

  'being parallel to the axis 36' of the lens 36.

  The deflection means 37 (see FIG. 2) consist of a plane mirror 37 ′ arranged in a plane perpendicular to the beam and inclined at an angle

  between 20 and 60 with respect to axis 21.

  The mirror 37 'deflects the beam 38 and focuses the deflected beam 40 at the focal point 39 on the parts to be welded formed in the example of application described here, by the exchanger tube 2 and a cylindrical sleeve 41 cooperating in joint with the internal wall of the tube into which it has been previously introduced and to which it is fixed, for example by expansion, or by hydraulic expansion or crimping. The deflection angles of the rays of the laser beam, between incident rays and corresponding reflected rays, are advantageously between 80 and 100. They are determined as a function of the inclination of the mirror relative to the axis 21 and of the distance existing between the mirror and

focus point.

  The deflected beam 40 constitutes a cone whose apex is the focal point 39. The deflection means 32 and the mirror 37 'are, for example, arranged so that the axis of the cone of the beam 40 forms an angle of incidence included between about 30 and 0 (normal incidence) with the plane tangent to the parts to be welded, which promotes good penetration of the weld. According to the invention, the welding head 9 comprises a clearance zone 42 separating the mirror 37 'from the

  focal point 39 of the beam.

  This internal clearance zone 42 or, which is considered equivalent, partially internal to the welding head, depending on whether the envelope 20 closes more or less above the mirror 37 ′, has a predetermined minimum section for passage of the gas emitted by the scanning means 14. This minimum section is advantageously greater than or equal to the internal half section of the cylinder 20, which is achieved when the beam deflection mirror is entirely located, relative to the focal point, at a distance substantially equal to, or greater than, the distance do between the axis 21 of the

  welding head of the focusing point 39.

  Advantageously, the distance d from the plane Pl parallel to the axis 21 and tangent to the upper edge of the mirror 37 ', or more generally containing the point of the deflection means closest to the focusing point 39, is provided for substantially equal or

greater than do.

  In the welding device according to the invention more particularly described here, the distance do is

around 10 mm.

  To allow access to the zone 42 located near one end of the head 9, of the gas injected into the head through the passages 25 arranged at the other end, the scanning means 14 comprise bypass means 43 of the gas around the focusing means 29. These bypass means are for example constituted in part by circular grooves hollowed out in the internal face of the cylinder

  envelope 20 of the welding head.

  A second cylindrical casing 44 for fixing the focusing means 29 and deflection 37 is also provided. This is carried out so as to allow the passage of the sweeping gas and cooperates contiguously with the casing 20 in which it can rotate with reduced friction. Holes 45 in the casing 44 in line with the grooves 43 constitute a second part of the bypass means. The gas follows the path indicated by the arrows referenced 26 on the

figure 2.

  Finally, the scanning means of the welding head 9 include deflectors 45 constituted by plane walls arranged symmetrically with respect to a plane passing through the axis of the welding head and fixed to one another by an edge longitudinal 47 to form a V-shaped angle whose internal surfaces are advantageously tangent to the beam 38 from the focusing means, as shown in Figures 2 and 7. An opening 46 is formed in the edge 47 to allow the laser beam to pass to

the parts to be welded.

  Another embodiment of the deflectors is shown in FIGS. 5 and 6. The deflectors are formed by curved walls symmetrical with respect to a plane passing through the axis 21 and thus constitute a constriction forming Venturi to channel and accelerate the gas to point 39 of

focusing and welding.

  As already mentioned, the welding device (see FIG. 2) comprises automatic rotation means (not shown) around the axis 21 of the welding head. These means are arranged to allow the rotation of the cylinder 44 and simultaneously of the various parts of the head which are fixed to it, namely the plug 24, the focusing means 29, the mirror 37 and the deflectors 45, inside the cylindrical casing 20 of the welding head. An annular device 50 fixed to the envelope 20 and comprising a seal 51 of known type (O-ring for example) seals with the envelope during rotation, so as to avoid leaks of the gas injected under pressure in the head 9 by

passages 25.

  Likewise, automatic longitudinal displacement means are provided. They allow the longitudinal displacement of the head in the tube to be repaired. They comprise a sealing part 52 substantially in the shape of a bell, the base of which is provided with a seal 53 with the tubular plate 4. The bell is traversed right through by the tubular casing 20 of the head. The bell and the envelope are movable relative to each other. They cooperate contiguously via the O-ring seal 54. The bell is kept under pressure on the tube plate 4 by means known per se (not shown). We will now describe the operation of the welding device according to the invention, applied to the repair of a small diameter tube and more particularly to the repair of an exchanger tube

nuclear reactor heat.

  The tube to be repaired is first identified, the repair sleeve 41 to be welded is then introduced into a predetermined location inside the tube, for example near the tube plate. The spider 15 carrying the welding head 9 is then put in place in the box 6, facing the tube to be repaired. The welding head 9 is then introduced into the tube 2 as far as a determined position relative to the sleeve to be welded, for example in abutment against the latter as shown in FIG. 2. This operation is carried out at

  distance and controlled by means 17.

  Furthermore, the interior of the tube is aspirated by means of the vacuum means 16, the sealed head 17 having been fixed beforehand on the other end of the tube 2 to be repaired, in the box 7. One injects then, under a pressure of a few bar, for example 10 bar, a sweeping gas for example a known neutral gas of argon or nitrogen, in the welding head; the gas enters the head via the passages 25, follows the path represented by the arrows 26 in FIGS. 2, 5 and 6 and comes to sweep under pressure (the injection pressure minus the pressure drops), the clearance zone 42. The deflectors 45 accelerate the gas in the zone 42 and orient its trajectory on the focusing point 39. The evacuation of the sweeping gas is facilitated by the suction from the interior of the tube 2, downstream of the head of welding, thanks to the vacuum means 16. The gas circuit having been put into service, a coherent radiation beam is emitted from the laser source 12 of the Nd + laser type: YAG,

  power on the order of a few hundred watts.

  The transmission is preferably carried out in pulse mode, the pulse transmission times advantageously varying from 5 to 100 ms, for example 20 ms, for an energy of 10 to 50 J, at a rate of 1 to 10 "strokes" per second, for example 6 strokes per second. The laser beam emitted by the source is condensed in a manner known per se by a condenser 12 '(FIG. 3), transmitted by the optical fiber 28, focused by the means 29 then deflected at 37 to be directed and focused under high incidence (close normal) on the sleeve 41 which is thus welded by fusion on the tube 2. The diameter of the focal point obtained with the device of the invention is reduced, for example of the order of 500 microns, and good weld penetration (e.g. 1.5

mm) is reached.

  During welding, there is emission of metallic vapors (vapor and liquid drops) in the direction of the mirror 37. The sweeping by a gas of the clearance zone 42 of minimum cross section greater than or equal to half the cross section of the head welding, minimum section for example of the order of 100 mm 2, makes it possible to expel the metal vapors outside the field of the mirror downstream of the tube and thus avoid pollution of the mirror, otherwise instantly covered by the recondensed metal vapors. The dynamic pressure exerted on the welding point also limits the very formation of

metallic vapors.

  The upper part of the head is then rotated around its axis to weld transparently continuously or discontinuously, the

sleeve to the tube which contains it.

  All these operations have advantageously been programmed and controlled by the control means 17. They are, for example, carried out either in mode

  automatic, or in semi-automatic mode.

  As it goes without saying, and as it follows moreover from the foregoing, the present invention is in no way limited to the embodiments only envisaged. It includes in particular the case where: - The rear part of the mirror 37 has a profiled bowl-shaped face to promote flow

  laminar sweep gas downstream of zone 42.

  - Other means of producing a bypass, focusing means, inside the welding head, are provided for the purging gas. This can for example be a pipe longitudinally crossing the welding head to inject the

  gas, directly in the scanning zone 42.

Claims (13)

  1. Apparatus for laser beam welding of two metal parts (2, 41) together, comprising an optical fiber (10) for transmitting the beam and a welding head (9) comprising means for focusing (29) of the beam from of optical fiber, characterized in that the welding head comprises deflection means (37) of the beam from the focusing means (29) for focusing the deflected beam on the parts to be welded, a clearance zone (42) separating the deflection means (37) and the focusing point (39) of the beam, and means for scanning (14) by a gas for scanning this clearance zone arranged to drive the deflection means (37) out of the field metal vapors from the fusion of metals during welding. 2. Welding apparatus according to claim 1, characterized in that the welding head (9) is elongated around an axis (21) substantially parallel to the direction of the axis of the laser beam from the optical fiber , and in that the beam deflection means (37) comprise a plane mirror   (37 ') for returning the harness to the parts to be welded.   3. Welding apparatus according to claim 2, characterized in that the beam deflection mirror (37 ') is entirely located, relative to the focus point (39), at a distance substantially equal to or greater than the distance do separating the axis (21) of the welding head of said focal point (39). 4. Welding apparatus according to any one of   claims 2 and 3, characterized in that the most   small distance separating the deflection means (37)   of the focal point (39) is less than 30 mm.   5. Welding apparatus according to any one of   claims 2 to 4, characterized in that   the end of the optical fiber (28) from which the laser beam comes is fixed in the axis (21) of the welding head (9), and in that the focusing means (29) comprise optical means ( 32) for deflecting the beam with respect to the axis (21) of the welding head, for directing said beam onto the mirror reference (37 ').   6. Welding apparatus according to claim 5, characterized in that the focusing means (29) comprise, arranged successively on the optical path of the beam coming from the fiber, a meniscus (30), a lens (31), convex plane, and the optical beam deflection means (32), said optical deflection means being constituted by a portion of spherical convex plane lens, obtained by cylindrical coring of said spherical lens, from an axis parallel to the axis of said spherical lens . 7. Welding apparatus according to any one of   Claims 2 to 6, characterized in that the means   scanning (14) by a gas comprise deflectors (45) constituted by walls arranged symmetrically with respect to a plane passing through the axis of the welding head and arranged to accelerate the speed of the gas in the clearance zone and increase the dynamic pressure exerted on the welding area. 8. Welding apparatus according to any one of   claims 1 to 7, characterized in that it   further comprises command and control means (17) of laser welding in pulse emission mode of the latter during successive emission times of duration between about 10 milliseconds and about 50 milliseconds. 9. Welding apparatus according to any one of   claims 1 to 8, characterized in that the source   of the laser beam used is a laser source solid of the YAG laser type.   10. Welding apparatus according to any one   of claims 2 to 9, characterized in that it   further comprises means for automatic rotation of the welding head about its axis and means for automatic movement of said welding head along its axis, arranged to longitudinally move said head relative to the parts to be welded. 11. Welding apparatus according to any one   of the preceding claims, characterized in that   the sweep gas is a neutral gas.   12. Method for laser beam welding of a metallic tubular sleeve (41) inside a metallic tube (2) with an internal diameter of less than about 50 mm, characterized in that: the sleeve (41) to be welded in the tube (2) with which it cooperates contiguously, - a welding head (9) of elongated shape is introduced around an axis (21) in the tube to a determined position of the head relative to the sleeve to be welded, - a beam of laser radiation is emitted in a fiber optic system connected to the welding head by focusing the beam of laser radiation from the optical fiber (10) on the sleeve portion to weld by means of deflection means (37) rigidly fixed to the head, which causes the sleeve to be welded to the tube by melting the metal, and - the welding area is swept simultaneously with the emission of the laser beam by a balyage gas injected inside the soda head ge and directed between the deflection means (37) and said welding zone, to entrain the metallic vapors coming from said melting of the metal outside the field of means of deflection.   13. A welding method according to claim 12, characterized in that the welding head (9) is rotated about its axis to produce a continuous or discontinuous circular welding of the sleeve (41), on the internal wall of the tube, the laser emission being carried out in pulsed regime, and the interior of the tube is vacuumed simultaneously by the end of the tube opposite the end through which the welding head was introduced, in order to promote the evacuation of the gas scanning neutral injected at the interior of said head.